A semiconductor device is manufactured by repeatedly performing a process which transfers a pattern formed on a photomask to a semiconductor wafer by lithography and etching. In this manufacturing process, the quality of lithography, etching and other steps, and the production of foreign matter, largely affect the yield of the semiconductor device, so in the semiconductor manufacturing field, it is important to have a method for early or prior detection when a fault occurs in the manufacturing process. In the prior art, a pattern inspecting and measurement apparatus employing an optical microscope was conventionally used, but in recent years, semiconductor devices have become more intricate while manufacturing processes have become more complex, so the use of electron microscopes is becoming more widespread.
One such appliance which uses an electron microscope is a circuit pattern inspection apparatus employing the Scanning Electron Microscopy (hereafter, SEM inspection apparatus). Many defects can be detected by this inspection apparatus such as electrical defects, adhesion of foreign matter, pattern shape defects, etc., and since an optical inspection apparatus cannot detect electrical defects, this special function of the SEM inspection apparatus is now attracting attention in the semiconductor manufacturing field. The detection of electrical defects in a semiconductor device by this SEM inspection apparatus is performed by charging a circuit pattern formed in a wafer surface, and using the contrast visualized by the charging. This is referred to as the voltage contrast method, and it is effective in detecting defective electrical properties of the semiconductor device.
Hereafter, the mechanism of forming a voltage contrast will be described using FIG. 2. FIG. 2 is a schematic cross-sectional view of a wafer in a step for machining a contact hole on a Si wafer, and embedding a metal therein. There is a normal part 401 in which the metal and Si wafer are conducting, and a defective part 402 in which the metal and the Si wafer are not conducting due to a residual film from defective processing of the contact hole. In order to detect this defect, the wafer must be electrostatically charged, a voltage contrast image obtained by taking the potential difference produced by the electrical resistance difference of the normal part and defective part as a difference in the number of secondary electrons detected by a detector 411, and the voltage contrast difference between the normal part and defective part measured. In the voltage contrast image, the wafer surface may be given a (1) positive charge or (2) negative charge according to the structure and inspecting conditions of the wafer to be inspected. The contrast of the pattern varies with the potential of the wafer.
To detect a defective electrical property using the aforesaid voltage contrast, and to detect a defect with high sensitivity, the wafer surface must be suitably charged. To obtain results which are highly reliable and reproducible, the electrostatic charge on the wafer surface must always be constant. Therefore, a method to measure the electrostatic charge on the wafer surface precisely is required.
Here, the potential measurement method by a conventional electron beam tester will be described. The schematic view of a prior art potential measurement method using an energy filter is shown in FIG. 3A. In FIG. 3A, 92 is a deflector, 87 is a first grid, 88 is a second grid, 86 is a sample, and the sample 86 is irradiated by an electron beam 81. The electron beam 81 can irradiate arbitrary points on the sample 86 due to the deflector 92. A potential 82 of this sample 86 with respect to earth is unknown. It is attempted to measure this unknown potential 82 by secondary electrons. Secondary electron 93 emitted from the sample 86 are accelerated by the first grid 87 to which a potential 84 of +10 to +100 V is applied, and most pass through the first grid 87. A potential 83 (energy filter potential) of, for example, −5 V is applied to the second grid 88. Secondary electrons 95 which pass through the second grid 88 are detected by a secondary electron detector 89. If the potential 83 applied to the second grid 88 is changed, for example to −30 to +30 V and the corresponding output of the secondary electron detector 89 is recorded on a XY recording waveform 91, a curve like A of FIG. 3B will be obtained. In general, this is referred to as an S curve. FIG. 3B shows the analysis characteristics of the energy filter obtained by the aforesaid operation. The horizontal axis is the potential of the second grid 88, and the vertical axis is the secondary electron detector output. Curves A, B are curves obtained for two different sample potentials. In both curves, the secondary electron detector output decreases as the potential of the second grid 88 becomes more negative. The curve A is shifted to the left-hand side compared with the curve B. This shows that the sample potential of curve A is a more negative potential. For the actual potential measurement, the output of the secondary electron detector 89 is set to the value shown by for example the arrow C, and the intersection points VA, VB with the S curve are obtained. This difference (VA−VB) becomes the variation amount of the sample potential 82. If the curve A is for the case of a sample potential of 0, (VA−VB) is the sample potential when B is measured (e.g., Scanning Electron Microscopy, Vol. 1, p. 375). To prevent the effect of fluctuation in the irradiation electric current of the primary electron beam, or the amount of secondary electron emission, there is also the method of differentiating the S curve shown in FIG. 3B by the second grid voltage, normalizing it, and calculating the variation amount of the sample potential 82 from the curve shift amount (e.g., JP 1986-239554 A).